In Reply.—It should be obvious to Dr Riva and his colleagues, as it is to everyone else, that a technique that permits absolute measurements of retinal blood flow in not yet available. That has never been our claim because we realize that our technique has most of the drawbacks and limitations of the two-point photometric systems, such as the one used by Riva and associates. With these facts in mind, I will try to clarify points raised in their letter.

We never stated in our report that we are measuring the first fluorescein molecules passing through the artery, nor was the phrase "first appearance time of fluorescein" ever used. What we are estimating is the time delay between two points of similar intensity from two time-fluorescence curves, which are detected at two separate points of the same artery. The points are close enough to avoid much distortion on